This invention relates to the field of purification of salt water by the humidification of air with that water, followed by dehumidification of the air to produce fresh water. The invention is especially useful for the continuous desalination of seawater in locales where the temperature of the ocean drops sharply with increasing depth.
Man's requirements for water are becoming increasingly evident as the earth's population increases, agricultural needs grow, and industries expand. Fresh water represents less than 3% of the water on earth. Of this 3%, nearly 75% is "trapped" as ice throughout the world, but predominantly at the polar ice caps. The remaining 97% of the earth's water is in the form of salt water or brackish water. The quest for fresh water has turned to desalination technology to convert the great oceans and vast inland brackish water reserves to fresh water.
Fresh water is obtained from salt water by separating the fresh water from an ever increasing concentration of salt water or by separating the salt from an ever increasing fresh water solution. Some of the processes which have been employed to accomplish this include simple evaporation, distillation, multieffect evaporation, multistage flash evaporation, thin film distillation, reverse osmosis, freeze crystallization, ionic separation and electrodialysis. Most of the existing desalination plants which employ these processes use fossil fuel as the energy source.
Solar distillation, which is an evaporation /condensation process, is based on the absorption of the sun's radiant heat on the dark coated bottoms of shallow trays filled with seawater (or brackish water). The water vapor which is formed is subsequently condensed on the cooler undersurface of transparent material located immediately above the pan. Convective cooling of the transparent cover by the atmosphere removes the heat of condensation. The transparent collector cover is sloped to allow the condensate to run off into collecting troughs at the base of the collector.
A variation on the solar still is illustrated in Hodges et al., "An Integrated System for Providing Power, Water and Food for Desert Coasts," 6(1) HortScience 10 (1971). In this system, a single stream of hot seawater is sprayed down through a packed tower countercurrent to a rapidly rising stream of air. This fills the air with a salt-free vapor which moves up through a duct into a second, condenser tower. Seawater enters the cycle at the bottom of the condenser and is pumped up within the tubes of a heat exchanger. Vapor forced over from the evaporator forms on the tubing as fresh water condensate and rains down to the base, where it is collected. The seawater spiralling up through the condenser and heated by the latent heat of vaporization is then conducted to a seawater heater, out of contact with the air stream, before entering the evaporator tower. The heat source for seawater heating is described as heat from a solar collector or waste heat from generators.
U.S. Pat. No. 4,172,767 to Sear describes a modified solar still which makes use of the difference in temperature between water on the surface of the sea and water at some greater depth. Seawater is collected in a tank and evaporated with the aid of accumulated rays from the sun. A blower forces the moisture-laden air above the heated seawater through a pipe into the depth of the sea. Moisture is condensed as the heat of condensation of the vapor in the air is dissipated in the seawater, and the potable water is then pumped up to the surface. Other patents relating to the use of ocean thermal gradients to produce potable water by condensation of water from a vapor-laden air stream include U.S. Pat. Nos. 4,186,311; 3,928,145; 4,151,046; 3,986,936; 2,820,744; 4,110,172; 4,187,151 and 3,257,291. U.S. Pat. No. 4,041,707 discusses cooling of warm surface air by passing it below the ocean surface, but does not suggest use of a moisture recovery system. Publications which pertain to compression of air using wave power or to related technology include U.S. Pat. Nos. 610,790; 875,042; 926,408; 1,267,936; 4,022,549 and 4,152,895.
Many of the processes currently employed for desalination are burdened with expensive requirements for mechanical equipment or energy from external sources. Evaporative desalination is characterized by high thermal energy consumption. While this consumption may be reduced by employing a multiple-effect concept, the number of effects employed in a particular operation is then determined by the trade-off of additional heat exchanger costs versus energy cost. In a vapor compression-evaporation process, approximately four-fifths of the energy requirement is for mechanical compression, and the remainder is for boiling water circulation. In the electrodialysis process, electrical requirements are proportional to the salt concentration of the water being purified, so the process is generally used only for desalting of brackish water. Separation of salt from saline water via an ion exchange resin is generally limited to small scale desalination projects since the chemicals required to regenerate the resins would become prohibitively expensive in large scale application.
No reference in the prior art envisions the combination of energy recycle and low operating energy requirements which characterize the inventive process.